1. Field of the Invention
The present invention relates to a method for manufacturing objects, starting from a powder, by pressing a preformed powder body at such a high pressure and such a high temperature that a density and bonding of the powder grains of almost 100% are obtained. The invention is particularly adapted for pressing materials which require a very high pressing temperature and which have shapes such that it is difficult to provide a capsule with the same shape as the body to be pressed. It is therefore particularly suited for the manufacture of bodies of special materials which are difficult to machine, such as silicon nitride, boron nitride, super-alloys, etc. The pressing can be performed in any pressure furnace in which the necessary temperature and the necessary pressure can be maintained.
2. The Prior Art
In U.S. Pat. No. 3,622,313 to Havel, there is described a method of manufacturing bodies of powder. Powder is filled into a glass capsule having the same shape as the finished product is to have, whereafter the glass capsule is evacuated, sealed, heated to softening and subjected to a high all-sided pressure in a pressure furnace. The density of the powder in the capsule is low and the shrinking during the hot pressing will be great. It is not possible to obtain the same density in different capsules, and therefore the dimensions of finished bodies may very within rather wide limits. The method does not make possible the pressing of bodies having very irregular or complicated shapes.
Patent application Ser. No. 400,049, filed Sept. 24, 1973, describes a method of manufacturing bodies starting from a powder, in which a body is preformed by isostatic extrusion so that a manageable powder body is obtained. Approximately the same density can be achieved during different pressings. By machining pre-pressed powder bodies, the desired measurements and complicated shapes can be exactly obtained. Because of the even density in the powder bodies, an equal degree of shrinking is obtained during a subsequent hot pressing and a great accuracy of measurement is thereby obtained in the finished products. According to this method, glass capsules are used which are so large that a clearance is formed between the capsule wall and the preformed powder body, and the glass capsule is allowed to acquire the shape of the powder body when it becomes workable after heating. However, materials for capsules which may be used at very high temperatures, for example glass with a high silica content for capsules for pressing silicon nitride requiring a pressing temperature of more than 1400.degree. C, are difficult to work and difficult to handle.